Niobium oxide, hydrous

Numerous methods have been described for the preparation of niobium(V) chloride, among them the reaction of niobium(V) oxide with thionyl chloride in a sealed system. In such a procedure some niobium(V) oxide trichloride, NbOCls, is almost always formed, and it is difficult to obtain the pentachloride completely free from this impurity, even by repeated sublimation. The simple, efficient method described here consists in allowing hydrous niobium(V) oxide to react with thionyl chloride at room temperature. Almost quantitative conversion is observed, the pentachloride dissolving in the thionyl chloride, from which it may be recovered, free of oxide trichloride, by vacuum evaporation... 

Niobium(V) chloride. Hydrous niobium(V) oxide (0.75 g. Nb) is precipitated from acid solution by the addition of ammonium hydroxide, thoroughly washed by centrifugation with water (two 15-ml. portions), 0.5 M nitric acid (two 10-ml. portions) to remove adsorbed ammonium ion, and acetone (three 20-ml. portions) and vacuum-dried at room temperature. If the initial hydroxide precipitation is carried out from hydrofluoric acid solution, an appreciable quantity of the hydrous oxide may dissolve in the nitric acid washes, presumably because of the presence of traces of fluoride. However, reprecipitation and treatment as above reduces losses at this stage. The dried hydrous oxide is placed in a 40-ml. centrifuge tube fitted with a standard-taper outer joint, and 10 to 15 ml. of freshly distilled thionyl chloride is added slowly, since the initial reaction may be vigorous. The vessel is stoppered loosely, and the reaction is allowed to go to completion at room temperature (24 to 48 hours). Any traces of undissolved hydrous oxide, usually very small, and any yellow crystalline compound (see Discussion) are removed by centrifugation, and the penta-chloride is isolated by vacuum evaporation of the thionyl chloride at room temperature and pumping for several hours at 10 mm. If necessary, the product is further purified by vacuum sublimation in a sealed tube ( 150°). The yield, based on dried hydrous oxide, is 90 to 95%. Anal. Calcd, for NbCU Nb, 34.39 Cl, 65.61. Found Nb, 34.27 Cl. 

The checker reports that standard filtration techniques can be substituted for the centrifugation steps. The reaction of hydrous niobium (V) oxide is done in a 100-ml. round-bottomed flask fitted with an outer standard-taper joint. The solution of niobium(V) chloride in thionyl chloride is filtered by attaching a filter tube (having a standard-taper inner joint at each end) with a medium-porosity sin-tered-glass filter to the neck of the reaction flask, attaching another 100-ml. round-bottomed flask to the opposite end of the filter tube, and inverting the assembly. The residue can be washed with small aliquots of thionyl chloride. With this kind of apparatus the synthesis can be scaled up by a factor of 10 or more. 

In addition to the obvious preparative advantages inherent in the pentachloride preparation, e.g., the low temperature of reaction and the possibility of performing the reaction without precautions against atmospheric moisture because of the protection afforded by thionyl chloride, the product is obtained free from oxide chloride. The major losses occur during the hydrous oxide precipitation and the nitric acid washings. The latter are essential to remove adsorbed ammonium ion, since, if this is not done, the reaction products will be niobium (V) chloride, in solution in thionyl chloride, and the bright yellow insoluble ammonium hexachloroniobate(V). In fact, the high purity of these two products in instances where complete removal of ammonium ion is not achieved shows clearly that the reaction of hydrous niobium (V) oxide with thionyl chloride is virtually quantitative. 

In an extensive review of the geochemistry of volatile-bearing minerals in mantle xenoliths, Ionov et al (1997) have pointed out that although minerals such as mica, amphibole, and apatite are often referred to as hydrous, in many cases they have very low H2O contents (Boettcher and O Neill, 1980). In such cases, these minerals may have significant amounts of fluorine, chlorine and CO2. Mica, amphibole, and apatite, together with the oxide phases, are important hosts for titanium, potassium, rubidium, strontium, barium, and niobium (Table 9). 

On being heated in acid solutions (or pseudo-solutions), niobium and tantalum hydrolyse and coagulate to form hydrous oxides. The following compounds may be used as collectors for traces of niobium or tantalum Zr(OH)4 and Mn02aq. in acid solutions, and Fe(OH)3 and Mg(OH)2 in alkaline solutions. When an alkaline melt (Na2C03, NaOH) is leached, Nb and Ta remain in the solid phase, while W, Mo, V, and Re pass into the aqueous solution [1]. 

Inoue, Y. et al.. Studies of the hydrous niobium(V) oxide ion exchanger. II. Affinity for various cations. Bull. Chem. Soc. Jpn, 58, 2955, 1985. 

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